The present invention relates to a satellite relay system, in particular, relates to such a system utilizing a time division multiple access (TDMA) system.
In the TDMA system, a plurality of earth stations can access a single satellite system on a time divisional basis by designating a particular time slot to each earth station. And by re-arranging the signals of each time slot in a satellite station, the satellite transmits the re-arranged signals to each earth station, and each earth station can be connected to any other earth station through satellite system.
As the size of antennas for communication satellite systems is enlarging, it becomes advantageous to use a spot beam antenna for directivity and efficient utilization of available frequency bands. The utilization of a spot beam antenna requires interconnection of each beam in a satellite system, and a scanning spot-beam system is one system which provides such interconnection.
A prior scanning spot-beam system has utilized a phased-array antenna. One of the prior satellite relay system utilizing a phased array antenna has been disclosed in "A Scanning Spot-beam Satellite System" in the Bell System Technical Journal, vol 56, No. 8, Oct. 1977, pp 1549-1560.
FIG. 1 shows a block-diagram of a prior phased-array type satellite relay system, and in the figure, E is the earth, S(#1) through S(#n) are earth stations and the circle surrounding each earth station shows the service area of each spot beam. The symbol A is an antenna mounted in a satellite station, and it should be appreciated that the spot beam of each antenna in a prior phased-array type satellite relay system is rather wide, and in order to obtain a narrow spot beam, that is to say, high antenna gain, a plurality of antennas, each of which is supplied a phase control signal, are utilized, then, by combining the wide spot beams of all the antennas a narrow spot beam is obtained.
In FIG. 1, the up-link signal, that is the signal from an earth station to the satellite station, is applied to the phase shifter(PH) through the antenna multiplexer(MUX) and the low noise amplifier(LNA). The antenna multiplexer (MUX) is provided for multiplexing receiving and transmitting operations in a single antenna, and the phase shifter (PH) operates to provide a narrow spot beam by shifting the phase of the signal of each antenna. The outputs of each phase shifter are combined in a single channel by the combiner(COM), the output of which is applied to the frequency mixer(MIX). The mixer(MIX) converts the frequency of the up-link signal to the down-link frequency according to the frequency of the local oscillator(LO), and the output of the mixer(MIX) is applied to the divider(DIV) through the amplifier(AMP). The divider(DIV) provides a plurality of channel signals by dividing the output power of the amplifier(AMP). Each of the outputs of the divider(DIV) is applied to said antenna multiplexer(MUX) through a phase shifter(PH) and a power amplifier(PA). The phase shifter(PH) operates to switch each down-link signal to a desired time slot and to provide a narrow spot beam for the transmission antenna. The output of the multiplexer is applied to the antenna, which covers the corresponding area on the earth. It should be appreciated that in the TDMA system, during the up-link the spot beam covers a single particular earth station, the down-link spot beam illuminates all the earth stations (#1 through #n), thus, communication between said particular station and each of the other earth stations is possible. That is to say, each earth station accesses the satellite station on a time divisional basis, and the switching between each earth station is performed in the satellite station itself.
However, a prior satellite relay system utilizing beam scanning by a phased-array antenna has the following disadvantages.
(a) Since the gain of the antenna system is proportional to the number of antenna elements, many antenna elements are required in order to obtain a narrow spot beam with sharp directivity. PA0 (b) Since the directivity of each antenna element is not sharp but is broad, the combined directivity of all the elements has a periodic grating lobe at the outside of a service area. PA0 (c) A digital phase shifter is generally utilized for controlling the exciting signal to each antenna element. However said digital phase shifter must have too many bits for realizing a sharp spot beam to an earth station, thus the structure of the control system must be complicated.
Further, it should be appreciated that a narrow spot beam antenna has not been utilized in a beam scanning satellite relay system since the high speed switching of high level signals is almost impossible, and said switching does not load the power amplifier.